February 19, 2005

Recycling Myths Pt.1

Recently, I got engaged once more in on-line discussion about recycling of post-consumer materials, particularly plastics. I'm getting tired of having to rebuke always the same fallacious arguments. So I decided to write about the problems - even myths - of polymer recycling in general. When people talk of plastics recycling, they generally refer to consumer plastics: those used for containers, packaging, bags etc. These plastics must be cheap and easy to use, and the life of disposable items is in the order of weeks - then they go in the bin. Engineering plastics are instead used for structural elements and other durable items, so in this latter case performance is (realtively) more important than price.

I thin it's advisable to begin with a brief primer on polymers in general. There are many different kind of polymers, but he most common ones for consumer products are polyethylene (PE), polypropylene (PP) - which have a very similar molecular structure, poly(ethyleneterephtalate) (PET), poly(vinylchloride) (PVC) and polystyrene (PS).But besides the chemical class, the mechanical properties of a polymer, and hence its suitability for a certain application, are determined by the mean molecular weight. This is in turn an expression of the length of the polymer molecules (or chains): as a general rule, the longer the molecules, the better the mechanical properties. Another important factor is the shape of these molecules, because long, stick-like chains pack more tightly than irregularly shaped ones.

And here lies a big rub: polymers nowadays are produced in big plants, using processes carefully optimized (by a meticulous R&D work) to to finely control the size and shape of the polymer chains. Thus any batch of PET (for example) coming out of a production plant has constant, defined composition and properties to satisfy customer's needs at a reasonable price.

Let's now examine a common plastic bottle for soft drinks. All over the world these bottles are mainly made of PET (a few places prefer PVC or a close relative of PET, poly(butyleneterephtalate), PBT): this PET is generally clear and transparent, and from a mechanical standpoint, it must have a high tensile strength in order to withstand the pressure that can easily develop inside the bottle; a stronger plastic will also allow for thinner walls, and thus less material to be used per bottle. Another appreciated property is impermeability to carbon dioxide, so that fizzy drinks will not lose their bubbles.Our bottle will also have a cap made of PP, and a label that can be paper attached to the bottle with a little glue, or plastic (PE, probably) tightly wrapped around it. The importance of these details will be clear later.

For now, let's suppose we have a thousand of bottles, all clear PET with no appendages to interfere, and we want to recycle them. First we have to carry them to a processing facility, and probably to do this we'll have to use a diesel-fuelled truck. A complication is that empty bottles, even if squased by hand, have a low density and thus require a large loading volume. This problem can be reduced by pressing the bottles in a tighter bale, but to do this we have to spend some energy (I worked at one of those presses, and it used a sizable electric motor, plus several liters of hydraulic oil). At the processing facility, the first thing to do is to shred the bottles in small pieces: this can be easily accomplished using dedicated shredders, but again that will cost some electricity.Then the PET chips must be washed: our bottles did not contain just water, but probably also fizzy drinks (basically, sugar). If these contaminants are not removed from the plastic, they will cause problems afterwards. Basically, the chips are immersed in a stirred tank of water - a lot of clean water. But water alone is not very good for cleaning; there are ways around it tho: an extremely turbulent flow of water will remove basically anything from the surface of the chips, but it takes energy to create such flow. Hot water can be used, but hot water does not come out of thin air (it is true that water for the washing stage may be heated using waste heat from other sections of the recycling process, but I'm not too sure about it). Detergent and/or a little of caustic soda can be added to the water, but these chemicals must be produced in some way, and also a rinsing stage is required to remove residues of detergent/soda. Then, the effluent water from the washing stage is contaminated and probably needs to be treated before being discarded to the sewers.

After washing, our PET chips must be dried and this requires some more energy. Update 27/02: I forgot to mention, before remelting the plastic has to be pulverized, and grinders for this purpose tend to require quite a lot of electricity. The next stage of the process is re-melting, and that's where the real trouble comes.PET for food applications must meet particular specifications regarding the release of maleodorating or toxic substances, and it must be free from impurities that can alter its colour and cause the appearance of defects in the final products. Re-melting occurs at about 280 C (prudeced by electric heating elements), a temperature that will char eventual organic impurities, causing the appearance of black specks. But that's not all: the high temperature alone is enough to cause some degradation of the plastic - rupture of the polymer chains in shorter pieces; this degradation can only get worse if water, acid or basic substances are present.The re-melting equipment is built and run to minimize the degradation of the processed PET, and it's even possible to add small amounts of chemicals that will re-join the extremities of broken chains, but that's far from an ideal solution. After re-melting, the plastic probably has to be filtered, and pushing something so viscous through a fine filter requires a sturdy pump. Finally, our PET is extruded, the thick filaments cooled and cut into pellets using electricity-driven machines. At the end of the day, every stage of the process must be run properly, otherwise we'll obtain a product with poor specifications, which cannot be used in high-spec applications and will thus have a lower market value.Even worse, the properties of our recycled PET may change unexpectedly, if the source material has properties different from expected - very dirty or contaminated with oily substances for example.

And this was just the almost-ideal case of clear PET, not mixed with other plastics. In practice, our bottles will always come together with some extraneous objects, and the bottles themselves have caps of a different plastic, that must be eliminated. Leaving aside manual selection and separation, that can be 100% selective but is also very inefficient, there are machines built for the purpose which use a variety of methods to identify the different polymers, both on whole objects and chips. The main problem with these machines is that, besides the usual energy comsumption, for each kg of impurities that are eliminated, at least another kg of good PET is lost as well. One nice idea is to use a specially designed centrifuge, where the highly turbulent flow will clean the plastic chips and separate them on the basis of density. Problem is, this machine is expensive to build and operate - but it does two steps of the process in one single operation. The biggest enemy of PET is PVC: at the melting temperature of PET, PVC will char producing black specks, and let off HCl that rapidly degrades the polyester. PVC also has density very close to that of PET, and it's thus very difficult to separate the two.

There is more to say about recycling, but this brief descritpion should be enough to demonstrate that everything is not as nice and easy as environmentalists say.Recycling of plastics is possible, but wether it is a bargain in term of energy and resources saving, and wether it is profitable or not, still has to be determined - case by case.

Comments are open to registered Blogger users only.Comments are tentatively open. Undesired comments may be deleted at the exclusive discretion of the blog administrator. However, the administrator does not necessarily endorse views expressed in the remaining comments.